Examples of molded article made of a synthetic resin material includes those using two different kinds of resins and having a layered structure in which one of the resins is used to form surface layers of a molded article (the innermost layer and the outermost layer in the molded article) and the other one of the resins is used to form an intermediate layer of the molded article. Further included are those having a layered structure in which a resin to exhibit a desired function is used to form an intermediate layer of a molded article and layers made of a resin for an exterior of the molded article are molded to interpose the intermediate layer therebetween to form a three layer structure. Not only multi-layer molded articles having such a layered structure using two kinds of resins to form three layers but also those having a layered structure using three kinds of resins to form five layers are commonly available.
Upon molding such a molded article having a multi-layer structure according to an injection molding method, methods have been known in which molten resins from two injection cylinders may be joined at a single nozzle portion of an injection machine and the joined molten resins are injected from the injection machine for molding. If a plurality of molded articles are simultaneously molded with this method, while the joined two kinds of resins are traveling from the nozzle of the injection machine through a runner portion of a mold to reach a mold cavity, the resin for the intermediate layer may be concentrated in part of the mold cavity due to an influence of the bending runner portion. As a result, the molded article is formed in such a shape where the intermediate layer is concentrated.
In contrast to the method in which joined multiple kinds of molten resins are sent out to branched runners in a mold from a single nozzle of an injection machine and then the molten resins are sent out to each of mold cavities, there is a method using a mold associating a hot runner nozzle with each of a plurality of mold cavities and utilizing an injection molding machine having a configuration in which an injection machine for injecting a molten resin for surface layers of a molded article and an injection machine for injecting a molten resin for an intermediate layer of the molded article are connected to the mold.
According to this method, the hot runner nozzle set to correspond to each of the mold cavities has the following configuration as described in Patent Literature 1, for example. In this configuration, nozzle resin passages to reach a plurality of resin discharge ports are each arranged in a concentric annular shape as a layout in a cross sectional plane in a nozzle diameter direction. Each of the resin discharge ports faces a single shut pin hole. Molten resins sent out from the respective resin discharge ports opened in a circumferential direction of the shut pin hole are joined in the shut pin hole and the joined molten resins as they are sent out to the mold cavity from a nozzle gate. Herein the respective resin discharge ports can be opened and closed by moving a shut pin back and forth through the shut pin hole.
The molten resins are separately flown into the respective resin passages in the hot runner nozzle from the branched runner portions in the mold. After the molten resin for surface layers of a molded article and the molten resin for an intermediate layer of a molded article, which requires a separate flow in the layered structure, are separately flown within the hot runner nozzle, these molten resins are fed into the shut pin hole from the respective resin discharge ports. The joined molten resins are then sent out to the mold cavity.